1. Field of the Invention
The present invention relates to an inorganic thin film electroluminescent device and a method for manufacturing the same and, more specifically, to an inorganic thin film electroluminescent device having a multi-layered insulating layer and a method for manufacturing the same.
2. Discussion of Related Art
An inorganic thin film electroluminescent device is such a device that an electron accelerated by high electric field collides with a phosphor to excite it, thereby inducing luminescence. While the inorganic thin film electroluminescent device has merits of high brightness, long life time, high resolving power, or the like, it has demerits of high driving voltage and a lack of a stable blue phosphor. It has been disclosed in “Journal of Applied Physics, 71, pp 1509, 1992”.
Meanwhile, the inorganic thin film electroluminescent device is composed of the phosphor for luminescence, an insulating layer for protecting the phosphor, and an electrode. Particularly, the insulating layer contributes to stabilize a device by protecting the device from dielectric breakdown and outer impurities, and to determine luminescent efficiency and luminance characteristic depending on an interface state between the phosphor and the insulating layer as well. It has been disclosed in “Applied Optics, 36, pp 545, 1997”. Therefore, the insulating layer should have a high breakdown field to contribute a stability of a device, and a high dielectric constant enough to lower a threshold voltage and to implement a device having a high brightness. In other words, a performance of the insulating layer is determined by figure of merit, which is obtained by multiplying a dielectric constant and a breakdown field. It has been disclosed in “Japanese Journal of Applied Physics, 36, pp 5696 1997”.
As the insulating layer for the inorganic thin film electroluminescent device, a low dielectric constant film (hereinafter, referred to as low-k film) such as a silicon oxide (SiO2), a silicon nitride (SiN), or the lime, which is focused on a stability of a device, was used in the beginning stage. Thereafter, an aluminum oxide (Al2O3) thin film having a relative dielectric constant of 8 to 10 was employed. Particularly, in the case of using a thin film deposited by an atomic layer deposition (ALD) method, figure of merit was the highest level of approximately 4 to 6 μC/cm2.
Then, a number of studies for enhancing breakdown field were performed by introducing a high dielectric constant film (hereinafter, referred to as high-k film). In the case of a titanium oxide (TiO2), it was obtained an improved value of 3.5 μC/cm2 from 1 μC/cm2, by doping cerium (Ce). It has been disclosed in “Japanese Journal of Applied Physics part 1, 36, pp 5696, 1997”. However, there was a problem from the point of view of a device stability, and a thickness of an insulating layer, i.e. approximately 270 nm, was thick relatively. Besides, many attempts to use a high-k film such as an yttrium oxide (Y2O3), a tantalum oxide (Ta2O5), a barium titanate (BaTiO3), etc. have been tried. However, there was a difficulty in insuring a stability and high performance of a device, in spite of its high figure of merit. Here, the high-k film refers to a thin film having a relatively high dielectric constant of 10 or more, and the low-k film refers to a thin film having a relatively low dielectric constant.
Meanwhile, in order to satisfy the aforementioned two contrary conditions, that is, high stability and high dielectricity, multi structures of Al2O3 and TiO2, or Al2O3 and Ta2O5 have been tried. The present inventors insured the high stability in the device, by employing an aluminum oxynitride (AlON) thin film. Here, the AlON thin film has a little bit improved permittivity and breakdown characteristic of approximately 10 MV/cm as compared with the conventional Al2O3, by using a plasma atomic layer deposition method. It has been disclosed in “Japanese Journal of Applied Physics part 2, 42, pp L663, 2003”. It may be a significant technology in that dielectric characteristics would be enhanced without lowering permittivity in the same material, by employing a new deposition method. However, it has been still required higher stability and permittivity in the device.